Chemical Composition of Planets

Hello, first post here. I'm trying to wrap my head around something here and I'm wondering if anyone can help. I've read that all terrestrial planets have basically the same chemical components - Iron, Silicon, Oxygen, etc - in roughly the same percentages. I also read that the composition of asteroids and meteors (the chondrite designations) aren't a general sampling of mineral content of planetary bodies but refer specifically to the particular bodies that broke up to form such meteors and asteroids. I also also read that in the case of the larger moons of the outer planets, minus the ice, are also made up of generally the same minerals.

Now, here's my question - when you hear about Io, you hear alot about sulfur, and when you hear about Titan, you hear alot about Nitrogen. Is that because these bodies have an abnormally high percentage of these materials, or that the particular conditions of these bodies - temperature, tidal stress, etc react primarily on these materials? For instance, if you put our Moon where Io or Titan is, you would see the same types of phenomena, or are these behaviors because of Titan or Io's unique chemical composition?

Staff: Mentor

The same overall composition does not have to mean the same fractions at the surface.

Earth has a lot of iron and even heavier elements, but most of it went down to the core at the time our whole planet was liquid. This did not happen for smaller objects, so their crust is richer in those materials (that's why some people think about mining asteroids).
And then there is chemistry. Chemical processes change the element distributions significantly even if the overall composition is the same for different objects.
Our moon does not have an atmosphere - putting it into the outer solar system now won't help, I guess, it would just stay dead.

Io is different from the other moons in the outer solar system - by being closer to the composition of the inner planets. Wikipedia has a description and the sources given there look nice.